This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 11-280343, filed Sep. 30, 1999, No. 2000-227319, filed Jul. 27, 2000; and European Patent Application No. 99125992.0, filed Dec. 28, 1999, the entire contents of which are incorporated herein by reference.
This invention relates to a nonlinear compensator which compensates for the nonlinear characteristic of the amplifier in, for example, a transmitter and is used in a transmitting system, such as a medium-wave, short-wave, ground-wave, or satellite transmitting system, or a cable television.
Presently, in analog television broadcasting, a predistorter having the opposite of the nonlinear characteristic of an amplifier makes nonlinear compensation. Since the nonlinear characteristic of the amplifier particularly varies with the operating temperature of the amplifier or the like, the compensation characteristic is changed, depending on the operating conditions of the amplifier.
In analog television broadcasting, the signal peak value is determined by a synchronous peak value and therefore is almost constant. Synchronization is maintained in the vicinity of the clip value. Therefore, phase distortion occurring in the vicinity of the clip level need not be taken into account and only synchronous amplitude has to be corrected so that the synchronization length may be the same. In addition, since the peak factor (peak value/average value) is relatively small, the linearity in the low-level signal region is not required much.
On the other hand, in a next generation of digital television broadcasting, use of an OFDM (orthogonal frequency division multiplex) system has been decided and various efforts have been made to put the OFDM system to practical use. In the OFDM system, because of the character of the OFDM signal, the peak factor is much greater than that in the analog television broadcasting. As a result, the linearity ranging from a low level to a high level is required in the OFDM system. Additionally, since the phase of each carrier plays a key role in information transmission, even a slight distortion in the phase rotation leads to degradation of the characteristic. Therefore, the nonlinear characteristic and phase rotation are required to be compensated for accurately.
As described above, in the digital signal transmission by the OFDM system, the nonlinear characteristic is required to be compensated for accurately. Since there have not been such needs in the conventional analog system, the development of a device that meets the requirements has been wanted.
It is, accordingly, an object of the present invention to provide a nonlinear compensator which is easy to adjust and capable of making adaptive compensation for the nonlinear characteristic and phase rotation that vary with season, weather, and time.
To achieve the foregoing object, a nonlinear compensator according to the present invention has the following characteristic configurations:
(1) A nonlinear compensator for compensating for the nonlinear characteristic of a compensated electronic device that deals with a transfer signal, the nonlinear compensator characterized by comprising: a signal processing section which takes in the input signal and output signal of the compensated electronic device, demodulates the signals suitably in such a manner that they have the same signal form, detects the time difference and phase difference between the two signals by correlating the two signals with each other, and not only synchronizes the two signals with each other but also causes the phase of one signal to coincide with that of the other on the basis of the detected time difference and phase difference; a distortion detecting section for detecting, as distortion components, an amplitude error and phase error in the input signal and output signal of the compensated electronic device that have been synchronized with each other and made in phase with one another by the signal processing section; and a distortion correcting section for producing the amount of nonlinear distortion compensation for the compensated electronic device on the basis of the distortion components detected by the distortion detecting section and compensating for the input signal to the compensated electronic device on the basis of the amount of distortion compensation.
(2) The configuration in item (1) is characterized in that the signal processing section detects the time difference between the two signals and synchronizes the two signals on the basis of the detected time difference in such a manner that the difference between the input signal and output signal of the compensated electronic device adjusted to the same signal form is minimized.
(3) The configuration in item (1) is characterized in that the signal processing section includes an input demodulation section for generating an input-side base band signal by demodulating the transfer signal to the compensated electronic device and an output demodulation section for generating an output-side base band signal by demodulating the transfer signal outputted from the compensated electronic device, detects the time difference and phase difference between the input-side and output-side base band signals obtained at the input demodulation section and output demodulation section, and not only synchronizes the two signals with each other but also causes the phase of one signal to coincide with that of the other on the basis of the detected time difference and phase difference, and further includes an output converting section for converting the output of the distortion correcting section into the format of the original transfer signal.
(4) The configuration in item (1) is characterized in that the signal processing section delays the input signal to the compensated electronic device according to the amount of delay equivalent to the detected time difference.
(5) The configuration in item (1) is characterized in that the signal processing section gives the amount of phase shift equivalent to the detected phase difference to the input signal or output signal of the compensated electronic device.
(6) The configuration in item (1) is characterized in that the distortion correcting section includes an amount-of-compensation table for holding the amount of distortion compensation, registers the amount of distortion compensation at the time of initial setting, reads the amount of distortion compensation corresponding to the amplitude of the input signal to the compensated electronic device from the amount-of-compensation table during service, and compensates for the input signal.
(7) The configuration in item (6) is characterized in that the amount-of-compensation table in the distortion correcting section includes a present-use table and a standby table, stores the amount of distortion compensation obtained at the distortion detecting section and, after the storage has been completed, replaces the present-use table with the standby table.
(8) The configuration in item (1) is characterized by further comprising monitor means for monitoring the compensated electronic device by monitoring the distortion components obtained at the distortion detecting section.
(9) The configuration in item (1) is characterized in that the signal processing section includes carrier synchronizing means for synchronizing the carrier frequency and carrier phase of the input signal to the compensated electronic device with those of the output signal of the compensated electronic device on the basis of the detected phase difference.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.